Prior research suggests that psychopathology in children and adolescents is associated with drug use disorders (DUDs), which are in turn associated with significant functional impairment and negative health consequences. However, the potential interacting influences of different types of psychopathology with family history and peer risk factors on the development of DUDs are poorly understood. The overall goal of this study is to examine whether the trajectory of DUDs during the transition from late adolescence to early adulthood is associated with childhood major depression (MDD) and conduct disorder (CD), family history of DUDs, peer drug use, and gender. Potential interactions of these factors with each other and with developmental stage (late adolescence through early adulthood) are of particular interest. Participants for this study will be drawn from the population-based sample of the Minnesota Twin Family Study, a longitudinal study of twins and their families. Participants were comprehensively assessed at age 17 and subsequently returned to the study at ages 20 and 24. The first specific aim of this study is to examine participants in four diagnostic groups--CD, MDD, CD and MDD, and neither disorder--to determine whether (and how) they differ in terms of the prevalence and development of DUDs from late adolescence through early adulthood. It is expected that these four diagnostic groups will differ in their patterns of DUDs. The second aim of this study is to examine how family history, peer drug use, and gender may interact with these forms of psychopathology across time. Specific hypotheses include: (1) family history of DUDs will be associated with DUDs in offspring; this effect is expected to be particularly pronounced at older ages and among participants with a history of psychopathology in childhood; (2) once family history of DUDs and child psychopathology are taken into account, peer drug use behavior is unlikely to aid in the prediction of DUDs; (3) by age 24, participants with a history of only CD or only MDD (and not the other disorder) will no longer be at increased risk for persisting DUDs unless another risk factor included in this study is also present; and (4) gender may moderate some of these relationships. These hypotheses will be examined using survival analyses, odds ratios, logistic regression, and hierarchical linear modeling; the correlated nature of the twin data used in this study will be statistically controlled.